1. Field of the Invention
This invention relates to piezoelectric ceramic compositions of lead titanate system.
2. Description of the Prior Art
As is known in the art, piezoelectric ceramic compositions include one which is composed principally of PbTiO.sub.3, one which is composed principally of Pb(Ti, Zr)O.sub.3, and one with Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3 and/or Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 dissolved in the solid state in said principal components as a second or a third component.
Of these, the binary or ternary system with Pb(Ti, Zr)O.sub.3 as the principal component are further given various additives to improve piezoelectric and electric characteristics, and these are used in pyroelectric devices, piezoelectric vibrators, resonators, oscillators, ceramic filters, surface acoustic wave filters and so on. On the other hand, a modified PbTiO.sub.3 system ceramic composition wherein Pb is replaced by another metal has a larger electromechanical coupling coefficient in thickness mode vibration than in contour mode vibration and a small dielectric constant (which characteristics are not found in the material composed of Pb(Ti, Zr)O.sub.3 system). As a result, where thickness mode vibration is utilized, the spurious vibration due to the contour mode becomes small, providing a merit that impedance matching with an external circuit is facilitated up to the high frequency region. However, since it is impossible to trap the energy of fundamental thickness expansion mode vibration near by the electrode, there is a defect that the fundamental thickness expansion mode vibration will not appear sharply. Further, another defect is that the temperature vs. resonant frequency characteristic is such that resonant frequency decreases linearly with increasing temperature. More particularly, the temperature coefficient of resonant frequency is between -50 and -100 ppm/.degree.C. This corresponds to the phenomenon of ceramics becoming softer with increasing temperature. Because of these defects, it has been very difficult to use modified PbTiO.sub.3 system ceramics wherein Pb is replaced by another metal for resonators, oscillators, ceramic filters and so on.